It is important to realize that in the operation of a laser seeker, two types of signal level variations exist. One of these is the variation of the average input signal level with range, and the other is a change in instantaneous signal level due to scintillation, foreground objects, etc.
Laser Seeker signal processors conventionally employed for proportional tracking utilize linear signal amplification of a type which limits the total instantaneous dynamic range to approximately 20 db, or .+-.10 db about the average pulse amplitude. However, the scintillation in the reflected laser energy from a target caused by missile and illuminator aiming motion can cause pulse to pulse amplitude variations exceeding 20 db. The resulting saturation or dropping of pulses will reduce the data rate and degrade guidance accuracy. In addition, terrain masking can occur, which is responsible for creating false pulses and preventing a large percentage of the energy from reaching the target.
In several instances, during field tests of laser illuminated tactical targets, a 25 db variation from pulse to pulse was observed due to scintillation and terrain masking. Under these conditions the 20 db instantaneous dynamic range of a conventional proportional processor will cause pulses to be lost with the resultant degradation in accuracy.
Further, with conventional processing equipment, it is possible to get a series of returns, such as from foreground bushes or other objects, which will have the effect in signal processors of limited dynamic range of causing the signal processor to track the false return. It will be seen that if a false pulse arrives earlier than the true pulse and is of a higher amplitude, if the false pulse is greater than 1/2 of the instantaneous dynamic range than the true pulse, then the system may lock upon the false target. If this type of situation is to be avoided, the signal processor must have a wide instantaneous dynamic range.